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I’ve been repairing an old Ampeg M15 – found a blown OT. I was told on the forum that any 6L6 push pull OT transformer would do and to pick an OT from similar wattage (30w) Fender circuit – so I did. Hammond sells a 4k OT for similar amps like a Vibrolux. ( Follow up conversations claimed that from the OT213 model number, the evidence points to a 6.6k OT. ) Sounds pretty darn good BUT, I ran the VTLADYI load line calculator and it implies that I exceed the max dissipation by a lot.

From the schematic pasted on the amp:
Vb+=390
Vscreen=360
Vbias =34V
Rcath = 250

I’ve attached screenshots at various OT impedance values and only the highest, around 7.8k falls within safe operation according to the calculator.

My questions are:
1) Will running at 4K damage the tubes (these are metal cans, not glass) – or do guitar amps ignore that curve?
2) The calculated cathode resistor, around 1.5K is well above the actual circuit of 250 ohms
3) Why is there such a variation in 6L6 OT’s?

Attached Images

	Screen Shot 2019-12-29 at 7.31.58 PM.png (199.1 KB)
	Screen Shot 2019-12-29 at 7.31.19 PM.png (182.9 KB)
	Screen Shot 2019-12-29 at 7.30.28 PM.png (175.5 KB)

Attached Files

Ampeg M15 schematic.pdf (1.83 MB)